Heat engines



Dec. 22, 1970 E. H. COOKE-YARBOROUGH 3,548,

HEAT ENGINES Filed Jan. 15, 1969 r 2 Sheets-Sheet l Flat Dem 1970 E. H.COOKE-YARBOROUGH 3,548,

HEAT ENGINES Filed. Jan. 15, 1969 2 Sheets-Sheet 2 L g5 g3 1 f2 24 3/ T7 United States Patent O ABSTRACT OF THE DISCLOSURE A Stirling heatengine has hot and cold variable volume chambers, which areintercommunicating through a regenerator, each formed at least in partby flexible walls.

Separation of the gas displacement and power output movement componentsof the movable diaphragms of the chambers is facilitated by permittingthe whole cold chamber to float and the regenerator, which providesmechanical connection between diaphragms of the hot and cold chambers,to move.

This invention relates to heat engines and is particularly concernedwith heat engines operating on the Stirling cycle and adapted, forexample, to drive an electric generator or to be driven to operate as arefrigerator.

BACKGROUND OF THE INVENTION The Stirling engine, which is areciprocating engine, is a very eflicient means of converting heat intomechanical power and vice versa and has been developed in the powerregion of tens of horse power to an efficiency of the order of 35%.

Such engines employ pistons sliding in cylinders and connecting rodswhich convert the reciprocating motion of the pistons into a rotarymotion.

Thermo-dynamically the Stirling cycle is a closed system and a problemwhich has been regarded as inherent in the system is that of sealing.One solution has been to provide a seal known as a roll-sock between theconnecting rod and the crank case.

Also, although conventional Stirling engines of tens of horse power havebeen operated at efficiencies of the order of 35%, if it is desired toscale down to a fraction of a horse power, the ratio of the area of thesliding surfaces to the power generated increases and the frictionlosses become significant in reducing the mechanical efficiency.

SUMMARY OF THE INVENTION The invention provides a Stirling enginewherein variable hot and cold volume chambers, which areintercommunicating through a regenerator, are each formed at least inpart by flexible walls capable of repetitive deflection for the life ofthe engine at the working temperature.

The Stirling engine is the only reciprocating engine in which theworking gas is ideally at the same temperature as the cylinder walls atall times. Thus the increase in the cylinder area/volume ratio resultingfrom scalingdown does not result in an increase in thermal losses fromthe gas to the cylinder walls. Elimination of sliding friction by use offlexible walls allows this advantage to be exploited at low powerlevels.

In one form of engine according to the invention the hot and coldchambers each have opposed side walls which are relatively movable byvirtue of the said flexible Walls, one of the opposed side walls of eachchamber is fixed, and coupling between the movable side walls and twoelectromechanical transducers is arranged for providing outputelectrical power from the engine and for im- Patented Dec. 22, 1970posing the necessary quadrature phase relationship between movements ofthe movable side walls.

Conveniently the coupling for imposing the said quadrature phaserelationship between the movements of the movable side walls comprisesan electrical coupling between the two transducers, which electricalcoupling includes a reactive load.

Preferably the coupling for imposing the said quadrature phaserelationship between the movements of the movable side walls comprises amechanical linkage for providing two separated movements correspondingrespectively to the in-phase component of movement of the movable sidewalls, which component is coupled to a displacer transducer, and to theout-of-phase component of movement of the movable side walls, whichcomponent is coupled to a power output transducer, and means forelectrically feeding a fraction of the power output, with appropriatephase shift, into the said displacer transducer.

In a preferred form of engine according to the invention the hot andcold chambers each have opposed side walls which are relatively movableby virtue of the said flexible walls, one of the opposed side walls ofone of the chambers, preferably the hot chamber, is fixed and the otherof the opposed side walls of that chamber is mechanically connected viathe regenerator to one of the opposed side walls of the other chamber,so that the regenerator and the side walls to which it is mechanicallyconnected can oscillate bodily to act a a displacer system.

In thihs arrangement output power is derived from the movement of theother side wall of the said other chamber, that is the side wall notdirectly mechanically connected to the regenerator, relative to the saidfixed side wall of the said one chamber. Some means is required to drivethe displacer and this could be an electrical means such as an externalsolenoid. Alternatively the displacer may be driven with the necessaryphase and amplitude by making one of the displacer diaphragms slightlylarger than the other and by suitable mechanical tuning of theoscillating displacer system. The electrical drive may be preferable ininstances where several engines are required to operate in synchronism.

In a further alternative arrangement according to the invention amechanical coupling is provided between the displacer system and theside wall from which output power is derived so as to provide power withthe necessary phase and amplitude for driving the displacer system. Thehot and cold chambers may comprise capsules of construction similar toaneroid capsules.

A feature of the invention is that the hot and cold volume chambers canreadily be in the form of thin disc shaped cavities in which the heattransfer characteristics are such that the ideal isothermal conditionsare closely approached.

The invention may with advantage be used with a radioisotope heat sourcethus providing self contained very long life mechanical or electricalpower generator. Moreover, if a comparatively large diameter thincapsule is employed to define the hot volume, a dilute or mixed fissionproduct source in contact with the whole of one side of the capsulebecomes a practicable possibility.

A moving coil system is an example of a linear electric transducer whichmay be driven by the engine. Other types which may be used withadvantage are moving iron and piezo-electric transducers.

Two Stirling engines in accordance with the invention may be mountedback to back with their mechanical terminals connected together toprovide a heat pump.

DESCRIPTION OF PREFERRED EMBODIMENTS Specific constructions of Stirlingengine embodying the invention will now be described by way of example 3and with reference to the accompanying drawings in which:

FIG. 1 is a diagram illustrating an engine and generator,

FIG. 2 is a diagram similar to FIG. 1 of a modification,

FIG. 3 is a diagrammatic longitudinal section of a further arrangement,

FIG. 4 is a diagrammatic representation of a modification of the engineof FIG. 1, and

FIG. 5 is a diagrammatic representation of a modification of the engineof FIG. 2 or 3.

Referring to FIG. 1 the engine comprises capsules H and C enclosing thehot and cold volumes respectively of the engine. The hot capsule H has acircular flexible metal diaphragm 1 and the cold capsule C has a similardiaphragm 2. The two volumes are connected by a gas transfer conduit 3in which is housed a regenerator 4.

Each of the diaphragms 1 and 2 is ocnnected by a rod 5 to a moving coil6 operating in the annular gap of a pot magnet 7 similar to the magnetand coil assembly of a moving coil loudspeaker. Thus, if the diaphragms1 and 2 oscillate they drive the respective coils 6 and an alternatingcurrent is generated in each coil. For the Stirling cycle, thediaphragms are required to operate approximately 90 out of phase andthis is achieved by suitable electrical interconnection of the movingcoils including a reactive load. The masses and restoring forces of eachmoving system are chosen such that their natural frequency ofoscillation is compatible with desired frequency of the electricaloutput. It is also necessary to provide a capacitative component in theload which will, in effect, be equivalent to a mass and will resonatewith the gas elasticity at the frequency of operation.

The engine operates to maintain the oscillations if heat is continuouslyapplied from a source to the capsule H and extracted by a sink from thecapsule C. When the diaphragms are oscillating 90 out of phase, gas isdisplaced to and fro between the hot capsule H and the cold capsule Cand there is a cyclic change of temperature and volume. The gas doeswork, which is given up to the electrical load (not shown) by expandingnearly isothermally in the hot capsule Hand abstracting heat from thesource. It will be noted that each diaphragm 1 and 2 functions both todisplace the gas and to generate power.

In practice it is desirable to position the electrical generatingcomponents at one end away from the heat source. This could be achievedby connecting the hot diaphragm to a linkage extending from the cold endbut this has the disadvantage of providing an unwanted heat conductionpath. An alternative arrangement is shown in FIG. 2.

In FIG. 2 the displacing and power functions of diaphragms areseparated. A power diaphragm 8 of a cold capsule C1 is connected througha rod 5 to a moving coil 6 and magnet 7 similarly to FIG. 1. The coldcapsule C1 also has a displacer diaphragm 9 to which one end of the gastransfer conduit 3, containing the regeneraltor 4, is connected. Theother end of the conduit 3 is connected to a displacer diaphragm 10 of ahot capsule H1. Extraction of heat from the cold capsule C1 isfacilitated by a heat sink 11.

The assembly comprising the transfer conduit 3 and diaphragms 9 and 10thus moves and functions as a displacer, causing the working gas to flowto and fro through the regenerator 4.

Since ideally no work is done on the gas by the displacer assembly andin practice very little, the mass and restoring forces can be chosen toallow it to oscillate freely near its natural frequency and by makingthe diaphragms 9 and 10 of slightly unequal area the periodic pressurechanges due to the gas displacement will furnish the small amount ofenergy required to keep the assembly in oscillation. The power diaphragm8 and the moving coil 6 respond to the pressure changes and are alsotuned mechanically and electrically to the same frequency and it can beshown that the displacer will oscillate with a phase displacement ofapproximately relative to the power diaphragm. This phase angle is notcritical, however.

In this embodiment it is preferred to resonate the gas elasticity with asuitable mass on the rod 5 which has the advantage over the capacitativeloading of FIG. 1 in avoiding electrical losses in the moving coils.

The embodiment shown in FIG. 3 operates on a similar principle to thatof FIG. 2 but incorporates a number of modifications and improvements.The hot capsule H2 consists of a rigid plate 11 formed on one side witha cavity 12 closed by a plane flexible diaphragm 13 clamped to the plate11 by a ring 14. The centre of the diaphragm 13 is supported on eachside by rigid discs 15 and 16 having central holes in which the transferconduit 3 is mounted. The peripheries of the cavity 12 and disc 15correspondingly chamfered such that when the diaphragm is flexed inwardsthe cavity is substantially wholly filled by the disc.

The cold capsule C2 is constructed similarly to the hot capsule H2 andis secured to the other end of the transfer conduit 3. Correspondingreference numbers have been given to the similar parts of the hot andcold capsules. The diameter of the cold diaphragm 13a is, however,smaller than that of the hot diaphragm 13 to provide the displacementenergy as explained above. A shaft 17 is secured to and extends from thecentre of the plate 11a of the cold capsule C2 and this shaft 17 issecured to bushes 18 carried by flexible spiders 19 supported from framemembers 20 to which the hot capsule H2 is also secured. Thus the coldcapsule C2 as a whole is flexibly mounted to oscillate along its axisand the reciprocating mechanical movement of the shaft 12 may be used toderive an energy output. As in the embodiment of FIG. 2, the mass ofsaid shaft 12 is arranged to resonate at the operating frequency withthe gas elasticity.

The mass of the cold capsule C2 provides only a limited heat sink anditself requires to be cooled. This is achieved by means of two aperturedplates 21 and 22 fixedly mounted on the frame members 20 parallel withthe disc 16a and back plate 11a respectively but just out of contact inthe extreme positions of movement of these parts of the capsule C2.

The elfect of the plates 21 and 22 is alternately to suck in and blowout air from the gap between themand the respective parts of the capsuleas the parts oscillate, thus providing a degree of forced cooling.

In operation, since the hot capsule diaphragm is larger than the coldcapsule diaphragm, when the pressure is high there is a force tending todrive the displacer assembly towards the cold capsule and the containedgas towards the hot cavity. The mass of the displacer assembly and therestoring forces of the diaphragms are chosen to make the systemresonant around cycles/sec. which is the chosen operating frequency ofthe engine. The resistance to movement of the displacer assembly at thatfrequency is in phase with the gas velocity, which leads the displacervelocity by 45. The force driving the displacer is the gas pressurewhich is nearly in phase Withl the movements of the output shaft 5. Withthe displacer tuned to resonance, the driving and resistance forces are:

in antiphase and thus the required 45 relationship is;

displacement is about cc. For a temperature difference of 340 C. thedesigned power output is 6 watts.

Extrapolation from data on possible diaphragm, materials indicate thatdiaphragms of a thickness of about 0.33 mm. made of Nimonic 80A orInconel X should be capable of at least 10 reversals (5 years at 50c.p.s.) at a temperature as high as 600-700 C. and at stresses at least50% greater than those assumed for the above diaphragm dimensions.

Power output can be substantially increased by operating with the gas athigh pressure say 100 atmos., but the diaphragm deflection would need tobe substantially reduced. The necessary displacement could then beachieved by the use of diaphragms in series.

Heat losses along the walls of the transfer conduit 3 require to beminimised and therefore the walls of the conduit are made as thin aspossible consistent with standing the axial force on the regeneratorcross-section corresponding, in the case of FIGS. 2 and 3, to the gaspressure on the effective area of the diaphragms.

The regenerator is required to be of high efficiency and to present theminimum of fluid friction. It may, for example, consist of a plurality,say 50, of stainless steel wire mesh discs 23 mounted within the conduit3 normal to its axis. A mesh of .4 mm. diameter wire at 2.5 mm. pitchwould be suitable. In the drawing a few of such discs are showndiagrammatically only for the sake of clarity.

In a modification the conduit 3 is replaced by a plurality of conduitseach containing a regenerator and dispersed over the area of the rigidcentral discs and 16 of the diaphragms.

In the above described embodiments the masses are unbalanced and theengine requires to be mounted on a massive base to take up the reaction.Although the massive shielding required of a ratio-active isotope heatsource could be used towards this end, it is preferred to provide abalanced system of masses thereby substantially eliminating any externalreaction. Thus, in a preferred embodiment, the mass of the moving partsof the capsules, output shaft and the moving components of thetransducer are reduced to a minimum and the masses required formechanical tuning are paired and arranged by means, for example, ofsuitable linkages, to move in unison in opposite directions thuscancelling any out-of-balance forces.

Additionally, or alternatively, the masses required to produce resonancein the displacer and/or output system may be reduced by introducing anelement of negative elasticity to counteract the natural elasticity ofthe sysd terns. Such an element may consist of a leaf spring prestressedin compression and adapted to move from a central unstable straightcondition to a bowed condition in either direction away from the centralposition.

The engine of the foregong examples has particular advantages whenadapted to operate with small oscillatory amplitude but in the case of amoving coil transducer a higher efficiency may be obtainable at higheramplitudes. It may thus be desirable to provide a velocity changingmeans between the engine and the transducer. If a linkage is providedfor mass balancing purposes, this linkage may be readily adapted toprovide the required velocity change. Alternatively the moving coil ofthe transducer may be connected directly through an elastic device suchas a helical spring to the engine and the coil provided with therequisite inertial mass to give the desired amplitude.

FIG. 4 illustrates a modification of the engine shown in FIG. 1. The twodiaphragms 1 and 2 move move approximately 90 out of phase. The motionscan be resolved into two components, an in-phase component which corresponds to the displacement of the gas between the two cavities atconstant volume, and an out-of-phase component which corresponds to achange in the sum. of the two cavity volumes.

In the arrangement of FIG. 1 as described above, these components areresolved electrically. Output power is derived from the out-of-phasecomponent (volume component) and a proportion of this is fed back intothe inphase component (displacement component) to maintain oscillation.Electrical resolution in this way involves problems of efficiency andpower handling capacity of the transducers and FIG. 4 illustrates anarrangement in which these problems are avoided by resolving the com- Iponents mechanically.

The diaphragms 1 and 2, and two electromechanical transducers, namely adisplacement transducer 21 and a power output transducer 22 aremechanically interconnected by a system of levers and links 23, 24, 25,26 pivoted at the points, marked by a cross, at 27, '28, 29 30, 31, 32,33, 34, 35, 36. In this example, all the pivots are provided byflexures. Pivots 28 and 35 are fixed to a support frame, as are thebodies of the transducers 21 and 22.

The mechanical resolution of the in-phase and out-ofphase components ofthe movements of the diaphragms may be appreciated by consideringfirstly, movement of the displacement transducer 21 with the powertransducer 22 locked; this will cause movement of both diaphragms in thesame direction corresponding to displacement of gas, and secondly,movement of the power transducer 22 with the displacement transducer 21locked; this will cause the diaphragms to move in opposite directionswhich corresponds to a change in total gas volume.

It should be noted that neither motion is entirely independent. Movementof the displacement transducer 21, even with the two cavities at thesame temperature, will generate a gas resistance force at the pivot 29which will be transmitted to the power transducer 22. Movement of thepower transducer 22 with the displacement transducer locked, whichcauses an up and down movement of the pivot 30, will also cause somerotation about this pivot 30 if the displacement transducer lever 24 isof less than infinite length. This would introduce a small amount ofdisplacement into the volume-change movement.

This arrangement of FIG. 4 will operate if some of the power output fromthe power transducer 22 is suitably phase shifted and fed to thedisplacement transducer 21. This arrangement would not be subject to thepowerhandling and efiiciency difficulties mentioned above, because theamount of power involved in displacing the gas is a relatively smallproportion of the total power output. There are, however, two quitesimple Ways of coupling the volume and displacement motions mechanicallyto obtain the desired phase relationship.

The displacement force should be approximately in phase with the gaspressure and in a sense tending to force the gas towards the hot cavity.One method is to put the pivots 30 and slightly out of vertical line, 30being to the left 35 in FIG. 4. A positive pressure in the two cavitieswill then have the effect of producing a net force tending to drive bothdiaphragms upwards, which has the desired effect of driving the gastowards the hot cavity. The same effect can be achieved by mass-loadingthe cold diaphragm. The acceleration of pivot 30 is proportional to thegas pressure; this added unbalanced mass will produce a couple aboutpivot 30- proportional to acceleration, which is in the right sense toforce the gas towards the hot cavity.

There is a possible refinement of the arrangement of FIG. 4 to reducethe mechanical forces on the walls of the regenerator which connectstogether the hot and the cold cavities mechanically. To achieve this thepivot 35, instead of being connected directly to the main structure ofthe assembly is attached to the centre of a bridge, the left-hand end ofwhich is supported on the stationary part of the hot cavity and theright-hand end of which is supported on a stationary pillar close to thevertical link 25.

The foot of this pillar is fixed to the base of the structure. For thisarrangement the upper rocker lever 26 and the bridge on which it issupported centrally at 35, act like a scissors configuration, resultingin a nearly exact balance between the forces applied to the hot cavity,and so largely relieving the regenerator of stress due to the pressureforce on the hot face.

With the moving regenerator configuration of FIGS. 2 and 3, inprinciple, a force of the required magnitude and phase to drive thedisplacer system can be obtained by making the cold diaphragm smaller inarea than the hot diaphragm. In practice, however, this may beinconvenient for several reasons. First, the cold diaphragm is in anycase more highly stressed than the hot one, so for a given power outputits diameter cannot very well be reduced. This means that the hotdiaphragm must be enlarged, and more lightly stressed, making the hotface larger than would otherwise be necessary. The second disadvantageis that the relative diaphragm areas have to be designed into the systemfrom the start and there is little or no prospect of adjustmentsubsequently. Third, for purely practical manufacturing reasons it ismuch more convenient to have hot and cold diaphragms identical in size.

With equal-size diaphragms the necessary force can be applied to thedisplacer system using a linkage as shown in FIG. 5. Two ore moreradially-disposed levers 41, 42 are connected via flexures 43, 44 attheir outer ends to the displacer system and via flexures 45, 46 attheir inner ends to the outjut shaft. Cold face plate 47 bears onintermediate points 48, 49. The force applied at 48, 49 is directlyproportional to the gas pressure. The proportion of this force which isapplied to drive the displacer is a/ b. The remainder is applied tooutput shaft 51. Clearly by suitable choice of the position of points48, 49 any desired force can be applied to the displacer and this forceis directly in phase with gas pressure. A tuning mass is indicated at 52and part of the regenerator is shown at 53.

The invention is not restricted to the details of the foregoingexamples.

I claim:

1. A Stirling engine comprising hot and cold variable volume chambersinter-communicating through a regenerator, each said chamber beingformed at least in part by flexible walls capable of repetitivedeflection for the life of the engine at the working temperature,non-positive coupling means between side portions of the hot and coldchambers, said side portions being movable by virtue of the flexiblewalls, the coupling means transmitting force for maintainingreciprocating gas displacement between the chambers, and the operatingcomponents of the en gine being tuned to resonate in correct phaserelationship in response to the forces transmitted by the couplingmeans.

2. A Stirling engine as claimed in claim 1, wherein the hot and coldchambers each have opposed side portions which are relatively movable byvirtue of the said flexible walls, one of the opposed side portions ofone of the chambers is fixed and the other of the opposed side portionsof that chamber is mechanically connected through the regenerator to oneof the opposed side portions of the other chamber, so that theregenerator and the side portions to which it is mechanically connectedcan oscillate bodily to act as a displacer system.

3. A Stirling engine as claimed in claim 2 wherein, when the engine isoperated to convert heat into mechanical energy, output power is derivedfrom the movement of the other side portion .of the said other chambernot directly mechanically connected to the regenerator, relative to thesaid fixed side portion of the said one chamber.

4. A Stirling engine as claimed in claim 2, wherein the said one chamberis the hot chamber, when the engine is used for converting heat intomechanical energy and is the refrigerated chamber when the engine isused as a refrigerator.

5. A Stirling engine as claimed in claim 4, wherein the area of therelatively movable opposed side portions of the said other chamber isless than the area of the relatively movable opposed side portions ofthe said one chamber, whereby gas pressure in the chambers provides theaforesaid coupling means for transmitting force for mainrainingreciprocating gas displacement between the chambers.

6. A Stirling engine as claimed in claim 2, wherein a mechanical levercoupling is provided between the displacer system and the side portionthe movement of which comprises the mechanical power movement, themechanical lever coupling providing the aforesaid coupling means fortransmitting force for maintaining reciprocating gas displacementbetween the chambers.

7. A Stirling engine as claimed in claim 1, wherein the movable sideportions of the hot and cold chambers are each coupled to anelectromechanical transducer for providing output electrical power fromthe engine.

8. A Stirling engine as claimed in claim 7, wherein the said couplingmeans for transmitting force for maintaining reciprocating gasdisplacement between the chambers comprises an electrical couplingbetween the two transducers, which electrical coupling includes areactive load.

9. A Stirling engine as claimed in claim 7, wherein the said couplingmeans for transmitting force for maintaining reciprocating gasdisplacement between the chambers comprises a mechanical linkage forproviding two separated movements corresponding respectively to theinphase component of movement of the movable side portions, whichcomponent is coupled to a displacer transducer, and to the out-of-phasecomponent of movement of the movable side portions, which component iscoupled to a power transducer, and means for electrically feeding afraction of the power transducer power, with appropriate phase shift,into the said displacer transducer.

10. A Stirling engine as claimed in claim 1, wherein at least one of thechambers comprise capsules of construction similar to aneroid capsules.

11. A Stirling engine as claimed in claim 1, wherein the chamberscomprise thin disc-shaped cavities.

References Cited UNITED STATES PATENTS 2,549,464 4/1951 Hartley 290-12,611,236 9/1952 Kohler et a1. 60 24 2,907,169 10/1959 Newton Q 60243,232,045 2/1966 Fokker 6024 3,339,077 8/1967 Shapiro 290-1 MARTIN P.SCHWADRON, Primary Examiner R. BUNEVICH, Assistant Examiner U.S. Cl.X.R. 6'2-6; 290 1

